Ceanothus Herbaceous) Growth and Reproduction

Am. Midl. Nat. 141:51±58 Effects of Fire, Browsers and Gallers on New Jersey Tea (Ceanothus herbaceous) Growth and Reproduction HEATHER L. THROOP1 Department of Biology, Carleton College, North®eld, Minnesota 55057 AND PHILIP A. FAY Division of Biology, Ackert Hall, Kansas State University, Manhattan, Kansas 66506 ABSTRACT.ÐWoody plant species in grassland ecosystems can be subjected to damage from ®re and multiple herbivore species, but interactions between ®re and herbivory can modify their separate impacts on woody plant life histories. We studied how galling (by Periploca ceanothiella, Lepidoptera: Cosmopterigidae), deer browsing (Odocoilius virginianus) and ®re affected the growth and reproduction of the woody shrub Ceanothus herbaceous (Rhamna- ceae) on a burned and an unburned site at Konza Prairie Research Natural Area in eastern Kansas. Fire was the major in¯uence on C. herbaceous growth, causing plants to produce long unbranched vegetative ramets from protected belowground meristems, while unburned plants were heavily branched and bore shorter shoots and numerous in¯orescences. Un- burned plants experienced higher gall frequencies, more galls on their longest shoots, but similar deer browsing compared to burned plants. Ramets with herbivore damage had more branches and in¯orescences than undamaged ramets, especially where both herbivores were present. Ceanothus herbaceous' ¯exible life history responses suggest tolerance of multiple forms of damage. INTRODUCTION Fire and herbivory exert pervasive in¯uences on the composition and productivity of many ecosystems, including the tallgrass prairies of North America. One major effect of ®re is suppression of woody plants in favor of grasses and forbs (Bragg and Hulbert, 1976). Fire typically suppresses woody species by destroying aboveground biomass and meristems, which alters subsequent plant growth, architecture, reproductive capacity and competitive interactions (Glenn-Lewin et al., 1990; Yeaton and Bond, 1991; Matlack et al., 1993). In addition, ®re modi®es abiotic parameters important to plant growth, including temperature, light and resource availability (Old, 1969; Hulbert, 1988; Niesenbaum, 1992; Turner et al., 1997). Herbivory has many of the same in¯uences on woody plants as ®re including biomass removal, meristem death and reduced growth, reproduction and competitive ability (Fay and Hartnett, 1991; Erasmus et al., 1992; Brown, 1994). The severity of herbivore impacts on plants depends on the timing, intensity and type of damage in¯icted, and a plant's postdamage growth and physiological responses (Watson and Casper, 1984; Rosenthal and Kotanen, 1994; Fay et al., 1996). Herbivore impacts may be compounded when plants are damaged by several herbivore species (Willis et al., 1993). Fire can strongly affect the extent to which herbivore damage actually occurs. For ex- ample, ®re affects which herbivore species are present (Warren et al., 1987). Some herbivore species tend to be killed in ®res because their phenologies or life histories render them unable to escape. Other species mainly experience indirect effects such as post®re 1 Present address: Department of Ecology and Evolution, State University of New York, Stony Brook, 11794 51 52 THE AMERICAN MIDLAND NATURALIST 141(1) changes in host plant quality and abundance (Stein et al., 1992) or abiotic conditions (Hulbert, 1988). These direct and indirect effects of ®res can affect herbivore abundances (Warren et al., 1987) and within-plant herbivore distributions (Rosenthal and Kotanen, 1994). As a result, the net impact on woody plant life histories of a ®re/herbivore damage regime can be dif®cult to predict. New Jersey Tea (Ceanothus herbaceous: Rhamnaceae) is found primarily on prairies and open woodlands throughout the midwest and south, and locally in the eastern United States (Bartgis et al., 1997). It is an abundant shrub in Flint Hills tallgrass prairies (Gibson and Hulbert, 1987), where it persists despite regular damage from ®re and herbivores. Fire damage typically occurs during spring prescribed burns, which are often conducted to re- duce woody species populations. In addition, two herbivores commonly damage C. herbaceous; a stem-galling moth (Periploca ceanothiella, Lepidoptera: Cosmopterigidae) whose abundance is directly in¯uenced by spring ®res (PAF and D.C. Hartnett, pers. observ.), and a browsing mammal, white-tailed deer (Odocoileus virginianus). The abundance of C. herbaceous despite consistent pressure from ®re and multiple herbivores suggests that this species possesses effective tolerance mechanisms. To examine damage tolerance in C. herbaceous, we conducted ®eld studies of plant growth characteristics in burned, galled and browsed plants in natural populations. We focused on these speci®c questions: (1) how does ®re affect the incidence of galling and browsing, and within-plant gall distributions? (2) how do ®re, galling and browsing affect plant growth form and reproduction? METHODS Study site description.ÐCeanothus herbaceous populations were studied at the Konza Prairie Research Natural Area, a 3487 ha tallgrass prairie in the northern part of the Kansas Flint Hills. Konza is divided into replicate watershed-sized experimental units assigned to factorial combinations of grazing (by cattle or bison) and burning (at 1, 2, 4, 10 or 20-yr intervals; see http://climate.konza.ksu.edu for detailed site map and descriptions). Konza's topography is steeply dissected, with each watershed containing shallow-soiled uplands, steep rocky slopes and deep-soiled lowlands. The overall vegetation consists of a matrix of dominant warm-season grasses (primarily Andropogon gerardii, Sorghastrum nutans, Panicum virgatum and Schizachyrium scoparium), numerous forbs and several woody species (Freeman and Hulbert, 1985; Van Cleve and Martin, 1991). Plant community composition at a speci®c location depends strongly on its particular ®re/grazing regime and topographic position (Gibson and Hulbert, 1987; Hartnett and Fay, 1998). Natural history.ÐCeanothus herbaceous grows abundantly on watershed slopes at Konza (average cover ù 12% on LTER vegetation transects) regardless of ®re frequency. Ceanothus herbaceous plants (i.e., genets) consist of multiple ramets originating from belowground meristems. During the ®rst year of growth, each ramet is generally unbranched, usually 20± 25 cm tall, and bears numerous lateral meristems. In subsequent growing seasons, ramets become branched due to new shoots arising from the previous crop of lateral meristems. After several years, each ramet supports many shoots and can reach over 50-cm tall. Spring ®res periodically reset plant growth form by killing all accumulated growth. This is followed by the rapid development of a new set of unbranched ramets. Ramets produce leaves in mid-to-late March, ¯ower during April and May and set fruits by mid-June (Great Plains Flora Assoc., 1986). Periploca ceanothiella is an abundant stem galler on Ceanothus herbaceous. Beyond basic taxonomic information (Cosens, 1908; Hodges, 1978) the species is unstudied. Galls form in July and contain one larva which feeds and overwinters in the gall, emerging the follow- ing March. Galls persist on the plant after larval emergence (PAF and HLT pers. observ.), 1999 THROOP & FAY:NEW JERSEY TEA 53 allowing quanti®cation of past galling history of plants. Morphologically, P. ceanothiella galls develop through a combination of inhibited internode elongation and lateral swelling. Galls are elliptical and retain the lateral buds from each node involved in the gall. Gall formation often prevents further terminal growth, and lateral shoots may arise from the gall's lateral buds. White-tail deer browse Ceanothus herbaceous primarily during fall and winter, removing the terminal meristem from varying numbers of shoots per ramet. The amount of tissue lost to browsing cannot be determined after-the-fact, but the loss of the terminal meristem may alter C. herbaceous growth by increasing lateral branching at the expense of additional terminal growth. This is a common effect of herbivores in other woody plant species (Whi- tham and Mopper, 1985). Field studies.ÐWe conducted ®eld studies during June and July 1995 in two ungrazed watersheds with similar soil type and topography. One watershed has been burned every April since 1988 (Konza designation K1A, 100 ha), and the adjacent watershed (K20A, 90 ha) has been burned approximately once every 20 yr, and was last burned in 1991. These watersheds are typical of burned and unburned Konza watersheds in overall species composition, soils and presence of C. herbaceous and its herbivores. To assess Ceanothus herbaceous herbivory rates, growth characteristics and reproduction, we counted the numbers of herbivore damaged shoots, intact shoots and in¯orescences on each ramet on plants in both watersheds. Plants were selected by marking four randomly- oriented transects per watershed, and then selecting the ®rst ten plants per transect (n 5 40 plants per watershed, except n 5 25 plants for browsing data in K20A). For each plant we noted the numbers of in¯orescences and undamaged, galled, or browsed shoots on each ramet. Herbivory was expressed as the percentage of ramets per plant galled or browsed. Data analysis.ÐWe used a randomization procedure (Manly, 1991) to analyze burning effects on herbivory rates and plant growth and reproduction responses. This procedure was chosen because standard analysis of variance techniques are considered inappropriate when samples

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